Stored cryogenic refrigeration systems are well known in the refrigeration industry. In general, these systems involve the use of a relatively large amount of refrigeration at cryogenic temperatures which is supplied on an intermittent basis by establishing a low temperature coolant reservoir of solid cryogen which can be economically created during a time period when there is low usage or the cost of electricity is lower. Buildup of refrigeration capacity in the reservoir can be accomplished relatively slowly, requiring fairly low power demands and relatively small capacity equipment. When the need for refrigeration arises, cold liquid cryogen is supplied at the necessary rate while taking advantage of the immediate availability of the capacity of the low temperature solid cryogen reservoir to remove the absorbed heat from a fluid stream returning to the reservoir. Such stored cryogenic refrigeration systems are described in U.S. Pat. No. 4,224,801 and 4,127,008, both to Tyree, Jr.
As indicated, stored cryogenic systems involve the use of mixtures of liquid and solid cryogen. The system generally consists of an insulated storage vessel containing a quantity of liquid cryogen, a gas compressor, and a liquid condenser. By using this equipment in a closed cycle, mechanical refrigeration can be stored by the production and accumulation of solid cryogen in the storage vessel. This stored refrigeration is recovered by recirculating liquid cryogen from the storage vessel through an external thermal load by means of a heat exchanger. The heated liquid cryogen and any gases produced are returned to the storage vessel and cause the solid cryogen to melt. This concept of energy storage relies on the heat of fusion which is the amount of heat required to change a quantity of solid to its liquid phase.
In such liquid-solid cryogen storage systems, it is highly desirable to be able to measure, on an intermittent or continuous basis, the solid fraction of the mixture which is a direct indication of the amount of stored refrigeration available. It is difficult to accurately determine the solid fraction of the mixture by visual techniques or by using floats or sonar, since a reliable solid to liquid interface is seldom achieved. Methods that require monitoring or analysis of solids content by doppler or density techniques are generally unsuitable since these techniques require a high degree of mixing and homogeneity of the vessel's contents.
The present invention provides a simple and reliable method and apparatus which can be used to determine the fraction of solids in a slurry or mixture of liquid and solid cryogen in a closed cycle incorporating a storage vessel.